In the commercial printing industry, an important step in the preparation of images for printing is the transfer of image information to an image recordable material that can be used repeatedly to print the image. While the image recordable material can take a variety of forms, one common form is the printing plate that includes a surface that can be modified in an image-wise fashion. Printing plates can take different forms. In one embodiment the modifiable surface includes a special coating referred to as an emulsion. An emulsion is radiation sensitive coating that changes properties when exposed to radiation such as visible, ultraviolet, or infrared light. An emulsion can include one or more layers that are coated onto a substrate. The substrate can be composed of a variety of materials such as aluminum, polyester or elastomers.
The transfer of image information to an image recordable material can be done in a variety of methods. One method in which image information is transferred to an image forming material is by computer-to-plate (CTP) systems. In CTP systems images are formed on the modifiable surface of an image recordable material by way of radiation beams or the like generated by an imaging head in response to image forming information. In this manner, images can be quickly formed onto the image recordable material.
The advent of CTP technology is part of an increasing trend towards automation in the printing industry. The increasing use of information technology to create and distribute electronic and print publications, coupled with the more widespread accessibility of such technologies is contributing to a greater demand for shorter print runs and faster turnaround times. These changes, in turn, have contributed to a greater push towards automating all aspects of the printing process.
Automating the printing industry does present some special technological hurdles, however. In the case of printing plates used in CTP systems, some of these hurdles result from the delicacy of the modifiable surfaces of these plates. These plates are easily marred, and if marred, can create undesirable defects in the final printed product. Any attempt to automate the handling of printing plates must include measures to prevent damage to the delicate modifiable surfaces of the plates.
Measures used to reduce marring of printing plates during storage or transport, however introduce additional problems for automation. Unexposed printing plates are normally supplied in packages in numbers that can range from a few dozen to several hundred with slip-sheets interspersed between adjacent printing plates. Slip-sheets are used to protect the sensitive surfaces of the printing plates by providing a physical barrier between printing plates. The slip-sheets must be removed from the printing plates prior to imaging.
The automation of slip-sheet removal and storage presents a number of challenges. Slip-sheet removal is not simply a matter of moving a single sheet from a stack of similar sheets. In general, slip-sheets are made from materials different from those used for printing plates (e.g. paper) and in particular, from materials suitable for not damaging the modifiable surfaces of the printing plates. Separating a slip-sheet from an adjacent plate can be complicated when the slip-sheet becomes adhered to a surface of the adjacent plate by physical mechanisms that can include electrostatic attraction or the expulsion of air between the surfaces. These mechanisms can lead to multiple plate picks that can lead to system error conditions. Increasing plate-making throughput requirements complicate matters further by necessitating that the slip-sheets be removed at rates that do not hinder the increased plate supply demands.
Conventional materials pickers have typically picked and removed printing plates and slip-sheets sequentially from a media stack. For example, in some conventional systems, a slip-sheet is first picked from the media stack and moved to a disposal container. Once the slip-sheet has been moved, a printing plate is then picked and moved to subsequent station where it is processed (e.g. imaging in an exposure engine). In other conventional systems, a slip-sheet is picked and transferred to a disposal container after the printing plate has been secured and transferred to a subsequent process. In either case, the sequential picking and removal steps can adversely affect the overall system throughput times. Increased throughput times can also arise when additional efforts expended to secure an additional sheet that is adjacent to a given sheet that is being removed from the media stack. In such a case, these efforts are required to prevent the additional sheet from being removed accidentally along with the given sheet. Conventional methods have typically employed media cassettes with passive or fixed separation plates or toothed structures to attempt to separate an underlying adhered sheet when a given sheet is lifted out of the cassette. In these conventional methods, the separation of the underlying sheet needs to occur over a limited amount of travel dictated by the distance between the given sheet and the fixed separation plate as the given sheet is lifted out of the cassette. Further, if the underlying sheet has not been separated from the given sheet, these conventional separation methods cannot easily be repeated when the given sheet is lifted out of the cassette to a position wherein the fixed separation plates no longer contact the given sheet.
Some conventional systems attempt to remove slip-sheets and printing plates simultaneously from a media cassette and convey them to a second location to be separated. In these conventional systems, suction is drawn through a porous slip-sheet to secure an underlying printing plate. Different slips-sheets can have different degrees of porosity that can affect the picking reliability of the underlying plate.
Once a slip-sheet has been secured and separated from a printing plate, its reliable disposal presents additional challenges for automated media handling systems. Specifically, in a device designed to have a large number of printing plates on-line at any one time, the slip-sheets that are removed each time a plate is picked must be accumulated somewhere for disposal. Conventional plate-making systems have employed complex media handling mechanisms that remove and convey slip-sheets to containers such as slip-sheet holders. The reliability and throughput of the media handling system may be adversely affected when a picked slip-sheet must be additionally conveyed and deposited into a slip-sheet holder. Further, when slip-sheets are crumpled during the act of picking, separating, conveying or depositing them into a slip-sheet holder, the slip-sheets can occupy a significant volume that increases the size of the slip-sheet holder, thus adversely impacting the required footprint of the plate-making system.
The presence of slip-sheets can hinder automation associated with the processing of image recordable materials. Consequently, there remains a need for better methods and apparatus for storing slip-sheets removed from a media stack made up of an arrangement of image recordable materials and slip-sheets.